In fact, not only for DAW software, but also for all the programs that can host the so-called plugin (such as, for example, the software for live performances by pianists or for video editing).
Let me explain:
Imagine that you have purchased a DAW to produce electronic music. Inside, you have already set up a complete series of virtual instruments and effects.But, among these, there is none that allows you to emulate the specific analog synthesizer sound you want. How are you going to do it then? Of course, you will look for a plugin, perhaps made by another manufacturer that emulates exactly the sound you want.
Buy the add-on, install it on your DAW and, in an instant, you will have added a feature to your main software. There is nothing simpler (or, at least, it seems that way).
What are the differences between VST and VSTi?
Not all add-ons are the same, obviously. They could be categorized into three main families: virtual instruments, effects and tools.
The first category includes all the plugins that are, in fact, virtual instruments, regardless of whether they are synthesizers, samplers or hybrid instruments. The second includes dynamic processors and effects in the form of a plugin (from compressors, reverberations, modulation effects, speaker emulations, etc.). The third is composed of measurement instruments and utilities (from measurement plugins, tuners, analyzers, etc.).
Often these three categories are identified with the word VST, or at most with two words: VST and VSTi.
Actually, it is a trivialization and an incorrect classification, since the abbreviation VST indicates a specific complement format, not all the existing complements.
However, although in some cases incorrectly, with the abbreviation VSTi the complements belonging to the first category (virtual instruments) are listed, while the abbreviation VST indicates the complements that are part of the second and third (effects and tools) .